1. Field of the Invention
This invention relates generally to spinal osteosynthesis and more particularly, to a system for making connections from various types of anchors to the spinal rods themselves.
2. Description of the Prior Art
Many types of hooks and screws and rods and connectors have been devised for use in connection with spinal osteosynthesis. U.S. Pat. No. 5,005,562 issued Apr. 9, 1991 to Yves Cotrel shows examples of all of these devices. It shows screws 19 including the "threaded rod" portion 21 anchored in bone, and hooks 18 including the curved blade portion 1 hooked around bone. It also includes transverse rods 22 with hooks 23 on them and set screws for clamping the hooks 20 and 23 to the spinal rods. Some such devices use screws and hooks which must be slided onto the end of the spinal rod for installation. In the Cotrel patent, the hooks and screws are "top opening" and have, for example, a body 2 with two "side branches" 4 and defining between them a channel 6 with a rounded bottom 7 to receive the spinal rod 3 between them. A plug 8 is screwed into the thread 11 formed in the inner walls of the branches 4, thereby closing the channel after insertion of the spinal rod from the top, and having either a center point 12 or peripheral ring 13 or both projecting downward from the face of the plug to penetrate and lock on the spinal rod when the plug is tightened in the channel. A sawtooth thread pitch on the screw is used to avoid spreading the branches as the plug is tightened.
The type of hook and screw head shown in and described in the Cotrel patent, where the spinal rod can be inserted directly from above into the receiving channel is in a category referred to as an "open system", in contrast to the necessity in certain other systems for introducing the spinal rod to the hook or screw axially or endwise of the rod through an aperture in the hook or screw and then clamping them. U.S. Pat. No. 5,261,907 issued Nov. 16, 1993 to Vignaud et al. is another example of spinal rods anchored in open-headed pedicular screws secured to the rod by a locking screw 6, with annular element 9 around the diapason-shaped cylindrical head 5 and element 9 locked in place around the "branches" 5a and 5b of head 5 by the tendency of the branches to spread as the screw 6 tends to spread the branches. There has remained a need for a system using open screws and hooks secured to the spinal rods, easily fixable in position by the use of a set screw, but without the need for some kind of special threads for the set screw or some ring or other arrangement capturing the branches of the screw head. The present invention is addressed to that need.